Without limiting the scope of the invention, its background is described in connection with a linearised amplifier.
A wireless communication system may use a digital cellular standard such as the IS-95 protocol or the IS-136 protocol to define a common method of communication between a mobile telephone (mobile station) and a base station. The IS-95 protocol may use Code Division Multiple Accessing (CDMA) or the IS-136 may use Time Division Multiple Accessing (TDMA) signaling mode to utilize available channel frequencies. For example, the TDMA signaling mode, such as that used in a Global System for Mobile (GSM), defines a carrier frequency comprising eight TDMA channels having eight time slots with each time slot allocated to one mobile station within a geographic area. Each time slot carries either speech or control data in a burst form.
The wireless communication system may use a digital modulation scheme such as Amplitude Modulation (AM), Frequency Modulation (FM) or Phase Modulation (PM) to modulate a carrier frequency with speech or control data. The wireless communication system uses a transmitter to transmit the modulated signal. The transmitter comprises a linearised amplifier comprising a lineariser and a power amplifier (PA) to linearly amplify the modulated signal to be transmitted.
The lineariser used in connection with the PA may be used to reduce distortion generated by the PA. For example, the PA may generate distortion when the PA is required to operate at high power levels where the PA operates at or near saturation. The lineariser reduces distortion by compensating the modulated signal to offset non-linear characteristics generated by the PA. The lineariser used in connection with the PA introduces distortion into the modulated signal that is the complement of the signal distortion generated by the PA. The distortion generated by the PA negates the distortion introduced by the lineariser producing a near linear output characteristic.
Turning to FIG. 1, where a prior art linearised amplifier utilized in RF transmitters is illustrated and denoted generally as 10. Linearised amplifier 10 comprises an RF input 12 for receiving a modulated signal and an RF output 14 for supplying a linearly amplified modulated signal. Linearised amplifier 10 further comprises a lineariser 16 and a Power Amplifier (PA) 18 disposed between RF input 12 and RF output 14. Lineariser 16 introduces distortion into the received modulated signal negating the affects of distortion when amplifier 18 is forced to operate at or near saturation during periods of high power levels. Lineariser 16 generates distortion based on input power level or distortion characteristics detected in a negative feedback circuit 20.
Linearised amplifier 10 as illustrated uses negative feedback circuit 20 to control the linearity of PA 18. However, although linearised amplifier 10 as illustrated utilizes negative feedback circuit 20, an open loop or closed loop controlled architecture may be used. For example, the architectural type of linearised amplifier 10 may be a Cartesian loop linear architecture, adaptive baseband predistortion architecture, Envelope Elimination and Restoration (EE&R) architecture or a Linear Amplification using Non-Linear Components (LINC) and its derivative Combined Analogue-Locked Loop Universe Modulator (CALLUM) architecture.
Although lineariser 16 may reduce distortion when PA 18 is operating at or near saturation, lineariser 16 may have a negative affect when PA 18 is operating under normal conditions. During normal operation of PA 18 lineariser 16 generates noise causing the noise figure of linearised amplifier 10 to increase. The increase in noise figure will significantly affect the quality of the modulated signal at normal operating conditions.
As may be seen, an improved apparatus to control unwanted of a linearised amplifier could improve the quality of the modulated signal to be transmitted.